Laser systems that use satellite platforms for earth remote sensing or communications will have doppler frequency shifts associated with the platform motion relative to the ground and the earth's rotational velocity, particularly with low earth orbiting (LEO) platforms, such as the space shuttle, space station, and polar orbiting platforms, that have a relative ground track speed of about 7 km/sec. The instant invention was developed specifically for a satellite-borne, atmospheric remote sensing instrument having a pulsed laser transmitter and optical receiver both located on a low orbit satellite, having a conical scan pattern centered about nadir and probing spectrally narrow atmospheric absorption lines. The scan period is ten seconds, and the pointing angle is 45 degrees off nadir, giving rise to doppler shifts of + and -7.3 GHz at near infrared wavelengths of 770 nm. The magnitude and direction of the doppler shift varies with the scanner azimuth angle and the satellite latitude. Although the instrument has a direct detection receiver, the optical filter bandpass is made very narrow in order to reject scattered solar background radiation; hence, the inherent doppler shifts would ordinarily be sufficient to move the laser frequency out of the filter bandpass. This invention provides for a means to simultaneously frequency tune both the laser transmitter and the optical receiver filter to compensate for the doppler shifts imposed by the relative motion between the satellite and the atmosphere, thereby allowing for the use of a narrow bandpass optical filter. This invention can also be applied to other satellite laser systems such as ground to low earth orbiting satellite systems and satellite to satellite optical communication links where there are large relative velocities which, in turn, induce doppler shifts that may be significant to the operation of the instrument.
The prior art for dealing with the doppler effect in laser satellite communication systems or laser satellite ranging systems has been to make the optical bandwidth of the receiver wide enough to encompass the frequency shifts and to increase the laser transmitter energy to overcome the higher solar background encountered with the wider filter. These techniques result in undesirable signal to noise characteristics and power requirements.